It's difficult to peer into the internal history of the Eurasian steppe and its largely pre-literate past for the early periods. So we're largely forced to rely on the accounts of the literate peoples around the periphery, whether that be Greek, Roman, Byzantine, Persian, or Chinese who dealt with and recorded the results of incursions from these outside peoples from the heart of Eurasia. For whatever poorly-understood reason, peoples seem to have boiled out of either Scandinavia or Mongolia periodically, pushing along the adjacent tribes, who pushed on their neighbors, in something like a butterfly effect, until this knock-on effect pushed a tribe over the border into the territory of some settled people.
Invasion by the "Germans" is the explanation Caesar gives for the migration of the Helvetii; it's debatable how accurate he was being, and how motivated to find political justification for his actions, but something induced them to pack up and move for greener pastures. A similar explanation may be found for the movement south through the Balkans and into Asia Minor of the Celtic tribes that became the Galatians. There is perhaps more evidence of this effect in the series of great migrations towards the end of the Roman empire, where the Franks, Burgundians, Vandals, and Goths all were forced westward in front of the Huns and Alans, who were themselves pushed westward by other steppe tribes of central Asia, perhaps ultimately terminating in the Xiongnu and their conflicts with Han China. There appears to be another wave of migration some hundreds of years later, corresponding with the migration of the Slavs, Bulgars, Avars and others into eastern Europe and the Balkans, the expansion of the Rus, the rise of the various central Asian Turkic khaganates, and the expansion of Tang China. Then there is of course the expedition of Batu and Subotai, in which atrocity and total war was a favored tactic, and only a succession crisis after the death of Ogodei Khan spared the rest of Europe. Then there is German colonization under the aegis of the Teutonic Knights, the crumbling of the Byzantine Empire under assault from the Turks on the one hand and the Venetians on the other, incessant warfare as the Ottomans expanded and contracted northwards through the Balkans, culminating in the early 20th century "Baltic Question" tinderbox as that empire fell apart and led to hundreds of millions of deaths throughout eastern Europe and atrocity on a scale to rival Genghis.
It's a history of blood and sword; not perhaps categorically different than the history of most places, but amplified by virtue of being the crossroads of so many moving peoples.